Characterization of Reactor Background Radiation at HFIR for the PROSPECT Experiment

This work describes an investigation of the background radiation present at the High Flux Isotope Reactor (HFIR) on behalf of the PROSPECT collaboration. The PROSPECT experiment is designed to make a precision measurement of the antineutrino spectrum at HFIR and search for sterile neutrinos. Temporal and spacial variation of neutron and gamma backgrounds at the experiment site for the PROSPECT detector are measured in order to determine if the reactor correlated radiation will contribute a significant background to the inverse beta decay signal. Knowledge of spacial background variation will also be used to inform the design of a local shield wall that is being built to mitigate gamma backgrounds entering the experiment hall. PROSPECT will be a movable detector, allowing it to measure at positions between 7 meters (near) and 12 meters (far) from the reactor core. While the reactor is on, we measured variation in gamma rates between 50% and 175% of the average at various positions throughout the volume of the near position. We find that the average gamma rate in the far position is 30% of the average gamma rates at the near position. Rate histories for neutron and gamma measurements are provided for entire reactor on cycles, and correlations are found between fluctuations in radiation and beamline operations below the experiment location. Simulation of the proposed PROSPECT detector along with its shielding package is carried out to determine the impact of reactor correlated backgrounds. We find an inverse beta decay event rate of 0.01 mHz within the region of interest from the ambient reactor on gamma field, compared to a 9.51 ± .70 mHz event rate from cosmic radiation. The backgrounds induced by a nearby materials irradiation experiment which may run during portions of PROSPECT's data taking are also investigated through measurement and simulation.